Amplifiers may be used in a variety of applications in wireless communication systems. For example, a base station may use an amplifier to amplify a downlink signal. Because wireless communication systems use radio frequency (RF) signals that exhibit high peak-to-average power ratios, amplifiers in such systems must be able to handle significant peak power levels above their average load. For example, amplifiers are most efficient when operating close to their maximum instantaneous output power. The efficiency of the amplifier decreases significantly as the output power level decreases from the maximum level. Thus, amplifier efficiency is a function of the output power level.
For a given power level (significantly lower than the maximum power level), an amplifier will be more efficient if the load impedance seen by the amplifier is high. However, maintaining the high load impedance when output power levels increase will make the amplifier saturate at a lower power level than originally designed. Saturation leads to signal distortion. When the amplifier operates in a low power regime, the load impedance is increased to improve the efficiency, and when the amplifier operates in a high power regime, the load impedance is decreased in order to avoid saturation. This load impedance modulation as a function of the signal power level is achieved by a Doherty amplifier. For instance, the Doherty amplifier improves amplifier efficiency by reducing the amplifier's saturated power level when the signal level is low, yet ramps up to full power capability when the signal peaks.
A Doherty amplifier has a main amplifier branch and one or more peak amplifier branches that are designed to enable the amplifier to provide high-power amplification by supplementing the amplification provided by the main branch during peak power operations. During normal power operations, the peak branches may be disabled such that high-efficiency amplification is provided by only the main branch.
Certain conventional design practices for Doherty amplifiers rely on Class-AB load-pull data and/or non-linear models in circuit simulators to characterize active devices. The design process is mainly based on manual calculations derived from Doherty design principles and limited load-pull data. Designers make many assumptions such as what input power split to use, which impedance modulation ratio to use, and which gain and compression values to use for each device. Such assumptions might not be accurate and may cause large discrepancies between expected and achieved performance characteristics of the resulting Doherty amplifier, leading to missed milestone deadlines and lower performance capabilities of the amplifier. The deviations from the desired performance are discovered only after the first prototypes are built and tested, at which point it is usually too late to make major changes to fix problems caused by faulty assumptions. Deviations that can be fixed involve manual tuning on the bench, which is a costly, time-consuming, and laborious process with no guarantees of success. Since the tuning is done on a small sample of prototypes, there is no guarantee that the solution will be optimal for large-scale production.
Moreover, rapid hardware design is important to expand to existing markets and penetrate new markets and generally requires a great deal of knowledge regarding the correct parameters to specify, equations to use, rules of thumb, etc.